The present invention relates to receivers for receiving signals having multiple signal spectra. Such a receiver may be particularly suitable for receiving signals from multiple satellite location systems, such as GPS (Global Positioning System) and Galileo.
When the Galileo system comes into operation, it will be advantageous for a locationing device to be able to make use of both the GPS and the Galileo systems. It may be that in some locations (e.g. when there is obstruction from objects such as nearby buildings) one system provides a better positional fix than the other. In locations where both systems can be received, it might be possible to improve the accuracy of a positional fix by integrating positional data from both systems.
The GPS system uses an L1 signal centred on 1575.42 MHz, and the Galileo system also uses a signal in that same area of the spectrum. FIG. 1 shows the GPS L1 spectrum (dashed line 1) and the corresponding Galileo spectrum (solid line 2). The energy of the GPS signals has a principal peak 3, with nulls on either side. The energy of the Galileo signals has two principal peaks 4 with nulls between and on either side of them. The principal peaks of the Galileo spectrum coincide with the nulls on either side of the principal GPS peak, and the principal GPS peak coincides with the central null of the Galileo spectrum. Thus, the principal energy of the GPS signal lies in a central frequency band 6 and the principal energy of the Galileo signal lies in two frequency bands 5 and 7 on either side of the central frequency band 6. The fact that both of these systems occupy a similar part of the radio spectrum means that there is potential for a receiver that is intended to receive signals from one of the systems to suffer interference from the other of the systems unless steps are taken to reject signals from the unwanted system.
A conventional GPS receiver might have a bandwidth of around 2 MHz centred around 1575.42 MHz, so that the peak 1 is passed for decoding. A corresponding Galileo receiver might have a bandwidth of around 4 MHz, also centred around 1575.42 MHz, so that the peaks 4 are passed for decoding.
If a satellite navigation device is to receive both the GPS and Galileo bands, then it could incorporate two receivers, one for each system. However, this would involve much duplication of components. A more efficient solution might be for a single receiver to have a filter that is adjustable so that the input bandwidth of the receiver can be set in dependence on the type of signals that are to be received. However, an adjustable filter is complex to implement and may have poorer performance than a static filter. Another option would be for the receiver to have two filters, either of which is switched into circuit depending on the type of signals that are to be received. However, using two filters occupies extra circuit board or integrated circuit area.
Accordingly, there exists a need for an improved mechanism for receiving GPS and Galileo signals in a single device. Similar considerations apply to other protocols whose signals are related in a similar way.